This invention relates to a high vacuum pump system of high pumping speed based on the cryo-sorption effect exhibited by adsorbents. The contemplated pump system is capable of evacuating large vessels within a very short time to final pressures of about 1 .times. 10.sup.-7 torrs and even less, residual gas atmosphere being entirely free from hydrocarbons.
A number of substances, such as zeolites, alumina or active charcoal, are known to possess very large surfaces of up to 1500 sq.meters/g, which enables them to adsorb large volumes of gases and vapours. At decreasing temperatures the adsorption capacity of these substances, particularly that of active charcoal, increases considerably.
An investigation into the adsorption properties of different active charcoals at 77.degree. K showed that after having been baked out at 100.degree. C they permitted final vacuum pressures of about 10.sup.-6 torrs to be achieved, and that by raising the baking temperature to 350.degree. C the vacuum pressure could be further improved to less than 10.sup.-8 torrs.
On the other hand further cooling of the active charcoal below 77.degree. K to 25.degree. K does not lead to a significant improvement in the obtainable ultimate vacuum pressure. However, at temperatures between 20.degree. and 15.degree. K the condensation and adsorption of neon and hydrogen results in a further lowering of the vacuum pressure. Disregarding for a moment the partial pressures of hydrogen and neon it therefore appears that an active charcoal which has been activated at about 350.degree. C and then cooled to 77.degree. K will allow residual gas pressures between 10.sup.-7 and 10.sup.-9 torrs to be achieved. These are vapour pressures which are comparable with the saturation vapour pressures attainable on cold surfaces at temperatures between 15.degree. and 20.degree. K. Since active charcoals may have surface areas amounting to something like 1500 sq.m./g. it is possible, by providing suitable quantities of active charcoal, to construct cryo-sorption pumps which combine high evacuating speeds with a sufficiently long service life to enable them to be employed in production plant. The activity of the charcoal can be maintained for periods which are the longer the better the fore-vacuum in the vessel that is to be evacuated. The charcoal can be reactivated by baking it out. In cryo-sorption pumps cooling surfaces of between 2 and 4 square meters can be easily provided for supporting quantities of between 500 and 1000 g. of charcoal, the equivalent of providing an active surface of between 0.7 .times. 10.sup.6 and 1.5 .times. 10 .sup.6 square meters.
In the case of sorbents hitherto used in vacuum technology thermal contact is usually so poor that neither the baking out temperatures of 300.degree. C required for full activation nor the working temperatures of 77.degree. K can be satisfactorily reached. This applies principally to loose bulk or to bonded sorbents and particularly in the construction of cryostats. Such adsorbents have therefore in the past been used exclusively for the purpose of maintaining residual gas pressures of between 10.sup.-4 to 10.sup.-6 torrs in closed vacuum containers (Dewar flasks) or of generating fore-vacuum pressures of around 10.sup.-2 to 10.sup.-3 torrs (sorption pumps).